1. Field of the Invention
The present invention relates to a method for manufacturing a metal graphite brush. More particularly, this invention relates to a method for manufacturing a metal graphite brush in which the amount of the graphite powder adhesives is adjusted in order to reduce the friction coefficient as well as to enhance the bonding strength of the metal graphite, which comprises the compounds of the brush.
2. Description of the Prior Art
A conventional typical method for manufacturing a graphite brush is disclosed in Japanese Laid-Open Patent Publication No. 90-51345. This patent concerns a method for preventing an excessive skin film on a commutator surface from being excessively formed as well as for reducing the abrasion on the brush during high speed rotations of large capacity, thereby prolonging the useful life-span of the brush. That is, the brush is manufactured with a mixture of powders of a aluminum abrasive, molybdenum disulfide(MoS.sub.2) and tungsten disulfide (WS.sub.2), each powder consisting of particles having diameters of less than 50 .mu.m, said powders combined in a solution of adhesives, prepared with grains of 70-300 .mu.m, artificial graphite powders 70 wt % mixed with adhesives of novolak phenol resin and methanol, pulverized and prepared with powders having a diameter 100 .mu.m, the compound of abrasives+lubricant+ adhesives of 0.5-15 wt % to graphite powder compressed at a pressure of 0.25 t/cm.sup.2 and fired at a temperature of 700.degree. C.
On the other hand, a general method for manufacturing a metal graphite brush for use in a small motor for a small automobile is different due to the addition of various metal contents according to the desired power output characteristic and motor durability. If high power output is desired, a metal content of 60-90 wt % is added. If high durability is desired, a metal content of 10-50 wt % is added. In addition, the graphite must be natural graphite in order to enhance the lubrication property. Furthermore, in order to improve the durability and the high power output of the brush and the noise reduction characteristics of the motor, control of the bonding strength and porosity is achieved by adjusting the amount and type of resins such as novolak phenol resins, resol type phenol resins and bisphenol A type epoxy resins, also called adhesives.
However, in manufacturing a metal graphite brush based on a conventional method, in which the metal contents are increased in order to improve the power output characteristics the specific resistance of the brush is reduced but a negative consequence is on the anti-abrasion of the brush and the noise reduction characteristic of the motor.
Further, after firing the brush, the addition of more adhesives for the purpose of improving the bonding strength achieves the predetermined bonding strength, but it causes the brush to become more dense as well as to become less porosity, so that the friction coefficient efficiency of the brush is increased which in turn generates a greater amount of noise in the motor. If the addition of different adhesives is desired in order to strengthen the bonding property, there are difficulties in that the differences in the carbon contents are dependent upon the kind of adhesives which restrict the smooth performance of the power supply function of the brush adapted to the commutator due to the porosity and the skin film of the commutator graphite and lubricants. Furthermore it has problems in that the adjustment of the amount of carbon, that is, hard carbon, to be contained in the adhesives removes the skin film and acts as an abrasive.
In order to resolve these problems and difficulties, methods have been used in a manner whereby the metal contents were relatively smaller to reduce the friction coefficient, natural graphite was used to reduce the fluctuations in the contact voltage, molybdenum disulfide is used as a lubricant to enhance the anti-abrasion, and one or two adhesives of phenol resin etc. are used to increase the bonding strength.